Both rendered and bare building constructions of mineral building materials such as those based on concrete or brick must, in many cases, be covered with a dense, firmly adhering coating to prevent corrosion of the building materials or of the reinforcing steel. Coatings are also required when chemical attack on the building material is to be expected or where structures such as floors, storage tanks and concrete containers need to be reliably sealed.
The film of coating must have sufficient elasticity to maintain the seal even if cracks form in the substrate, i.e., it should be capable of bridging over the cracks. The protective function of the coatings is ensured only if the coating films are sufficiently elastic and form a layer of sufficient thickness. These factors have been indicated in the literature (see, for example, "Rissuberbruckende Kunststoffbeschichtungen fur mineralische Baustoffe" by Gunter Rieche, Otto-Graf-Institut Stuttgart, Farbe und Lack, publishers Kurt Vinzenz Verlag, Hanover, Year 85, pages 824-831, 1979).
The coatings are generally required to have a minimum thickness of 0.1 mm and the coating materials should have a minimum elongation of 100%.
When choosing a coating material, it should be noted that the surfaces to be coated are generally porous and frequently contain a certain quantity of water, the so-called physically associated moisture being assumed to be approximately 3.5% by weight. In many cases, especially when applying coatings outdoors, the water content is found to be considerably higher, and this is bound to lead to interactions with the coating material.
Solvent-free polyurethane systems are the obvious choice for the desired thick-layered elastic coatings since the polyisocyanates and polyalcohols used as starting materials for the polyurethane coatings can be selected as required to provide elastic coating films of any thickness. Difficulties arise however in that the isocyanate groups not only react with polyalcohols but also enter into competitive reaction with the moisture of the underlying surface. This has the undesirable effect of splitting off carbon dioxide which causes the coating material to foam up and form blisters. Numerous proposals have been made aimed at the suppression of this undesirable side reaction. The addition of molecular sieve zeolites has proved to be particularly effective for removing any troublesome water content in the fillers, pigments or other constituents of the coating materials so that water derived therefrom will not cause blistering (see, for example, "Polyurethane fur Beschichtungen und Abdichtungen im Bauwesen", Hermann Gruber, Farbe und Lack, Publishers Verlag Vinzenz, Hanover, Year 80, pages 831-837, 1974).
The "molecular sieve zeolite method" cannot however meet the demands of excessive subsurface moisture with the result that the formation of blisters due to the splitting off of carbon dioxide cannot be prevented with certainty when the coating is applied to moist substrates.
The present invention provides a new process for the production of coatings and enables thick, elastic coatings to be applied, in particular to mineral based building materials containing water, which would reliably prevent the undesirable formation of blisters even in the presence of a high content of subsurface moisture, and for which solvent-free or low solvent coating compounds of low viscosity and sufficiently long pot life could be used.
This problem could be solved by the process according to the invention described in detail below.